1. Field of the Invention
The present invention relates to a scanning probe microscope. In particular, the invention relates to an electrostatic surface actuator operatively coupled to a scanning probe tip of the scanning probe microscope.
2. Description of Related Art
Scanning probe microscopes are known. For example, U.S. Pat. No. 4,724,318 to Binnig describes a method of imaging the surface of objects with atomic resolution. U.S. Pat. No. 6,005,251 to Alexander et al. describes a voice coil scanner for a scanning probe microscope, and U.S. Pat. No. 6,323,483 to Cleveland et al. describes a piezoelectric scanner for a scanning probe microscope. Known scanning probe microscopes use piezoelectric actuators to position the probe tip over the test surface, and such actuators transform the applied voltage into a displacement and are useful for ranges of motions from 100 μm down to 0.1 nm. Unfortunately, these piezoelectric actuators have limitations due to creep and resonant frequencies. When a voltage is applied, a piezoelectric actuator displaces to a corresponding position. The piezoelectric material “relaxes” at that position. When a second voltage is applied, the position corresponding to the second voltage exhibits a “memory” corresponding to the position when the initial voltage had been applied. This is referred to as hysteresis. The position of the piezoelectric actuator depends on the history of the applied voltage.
When scanning a probe over a surface, the scan speed is limited by several factors such as tip wear, sample abrasion, cantilever response time, detector sensitivity, software acquisition times, external vibrations, available scanning speed, and the acceleration available to accelerate the tip normal to the surface, among others. Much progress has been made in many of these areas. For example, a research group demonstrated high frequency tips for data storage applications. See Reid et al. (5 MHz, 2N/m Piezoresistive Cantilevers with INCISIVE tips, 1997 International Conference on Solid State Sensors and Actuators, Chicago June 1997, pp. 447-450). However, the acceleration available to accelerate the tip is still extremely limited, being on the order of only several times the acceleration of gravity (g) for conventional scanning probes.
When the piezoelectric actuator is used to displace a scanning microscope probe tip, the scan speed is limited. Operating at higher scan speeds results in the probe tip being driven more by mechanical resonance of the piezoelectric actuator than by the applied voltage. This limits the scan rate achievable with the piezoelectric actuator.
The large mass of the piezoelectric actuator has a compounding effect when several actuation stages are stacked on each other. For some scanning probe microscopes, it is desirable to mount the z-actuator on an x-y stage. The large mass of the z actuator reduces the scan speed that the x-y stage can produce. The large mass of the z-actuator also implies that a large inverse reaction force is applied to the x-y stage when the probe is accelerated. Although Cleveland et al. (U.S. Pat. No. 6,323,483) and Bartzke et al. (U.S. Pat. No. 5,524,354) disclose a balanced piezoelectric actuator to reduce the inverse reaction force in which two parts of the actuator move in opposite directions, if applied to a z actuator of a scanning probe microscope, the use of a balanced piezoelectric actuator in this role would double the size and mass of the z actuator, further reducing the scanning speed that the x-y stage can provide. Furthermore, control of such actuators can be difficult and renders the overall scanning probe microscope more complex.
Some attempts have been made to couple a scanning probe to a micromachined actuator. One approach proposed using a torsional micromachined element to allow fast out-of-plane positioning of either an AFM or STM tip. See Miller et al. (Scaling Torsional Cantilevers for Scanning Probe Microscope Arrays: Theory and Experiment, 1997 International Conference on Solid State Sensors and Actuators, Chicago June 1997, pp. 455-458). At present, scanning probe microscopes use many possible tips. As yet no group has demonstrated a micromachined structure that is capable of scanning standard tips. cl SUMMARY OF THE INVENTION
The invention improves the state of the art by providing an electrostatic surface actuator operatively coupled to a scanning probe tip. The electrostatic surface actuator positions and drives the scanning probe tip over the test surface.
This and other improvements are realized in a scanning probe microscope that includes a scanning probe tip and an electrostatic surface actuator operatively coupled to the scanning probe tip. The electrostatic surface actuator includes a movable member that has a first surface with a first plurality of electrodes disposed on the first surface and a stationary member that has a second surface with a second plurality of electrodes disposed on the second surface. The movable member is resiliently coupled to the stationary member so that the first and second surfaces are disposed in a confronting relationship and so that the movable member is capable of being displaced with respect to the stationary member in a first direction. The first and second pluralities of electrodes are configured to generate electrostatic forces in response to voltages applied to the first and second plurality of electrodes, the electrostatic forces being aligned to laterally displace the movable member in the first direction generally parallel to the first and second surfaces. The movable member is mechanically attached to the scanning probe tip such that the scanning probe tip is controllably positioned by displacement of the movable member.
This and other improvements are alternatively realized with a method of scanning a sample using a surface electrostatic actuator and this method includes mounting, displacing and sensing. The mounting mounts a probe on a movable member of the surface electrostatic actuator. A surface of the movable member is generally disposed to face a surface of a stationary member of the surface electrostatic actuator. The displacing displaces the movable member relative to the stationary member in a direction generally parallel to the surface of the movable member to scan the probe over the sample. The sensing senses a property of the probe that is responsive to the scan of the probe over the sample.